1. Field of the Invention
The present invention relates to shape memory polymers and, more specifically, water responsive shape memory polymers.
2. Description of the Related Art
Shape memory polymers (SMPs) are a class of stimuli-responsive materials that can be elastically deformed and subsequently fixed into a temporary shape by network chain immobilization, and later recover to their original (permanent) shape when exposed to external stimuli that re-mobilize the network chains. Direct heating is the most widely studied external stimulus to induce shape recovery in the past years. Other stimuli such as light, electricity, magnetic field, and moisture have also been utilized as the recovery triggers.
Compared with heat-triggered SMPs, water responsive SMPs are capable of regaining their original shapes simply by immersing the samples in water. Here, undesirable effects resulting from external heating, such as damage of surrounding tissue and cells when activating a smart implant, can be avoided. In 2003, it was accidentally found that a pre-deformed and fixed film made of a commercially available polyurethane (Tg=35° C.) became rubber-like after one month of exposure in air at room temperature and recovered its original shape, with the Tg decreasing to about 22° C. Later, moisture was identified as the stimulus causing the polymer to become rubber-like and thus triggering the shape recovery. This research also unveiled the recovery mechanism: water molecules, which diffuse into the polymer sample, disrupt the intramolecular hydrogen bonding and mobilize the previously vitrified network chains, thereby shift SMP transformation temperature (here, Tg) to lower temperatures and allow for room temperature actuation. The shape-memory effect associated with the lowering of transition temperatures has also been shown for polyurethanes composites with carbon nanotubes. In all cases, the shape memory effect was slow, with recovery taking at least 140 min.
A different strategy for water-induced shape-memory polymers has been realized by incorporating a hydrophilic or water swellable component into the structure. In this way, the shape recovery can be greatly accelerated. The water-activated SM effect has been demonstrated in poly(ethylene oxide) (PEO)-based polyurethanes with the hydrophobic polyhedral oligosilsesquioxane (POSS) moiety as the hard-segment. Exposure to water resulted in the water-swelling of the PEG segment and recovery of the permanent shape. The polymer films, obtained by solution casting, showed incomplete recovery (65-85%) after 300 seconds at 35° C. water. By modifying chitosan with PEG and epoxide crosslinking, a water-activated biodegradable stent has been prepared. The equilibrium shape was chemically fixed by crosslinking. The raw materials (chitosan and polyethylene glycol) used were relatively hydrophilic, and a subsequent immersion in water led to rapid hydration and recovery in a short period of 150 seconds. Shape memory polymers sensitive to organic solvents can also be obtained which are similar to the hydrophilic SMPs that are sensitive to water. It has been observed that dimethyl formamide (DMF) is capable of activating the shape memory response of a styrene-based SMP. Again, however, this effect was slow, with recovery taking at least 180 minutes. The slow response of existing water-triggering shape memory systems has thus indicated the need for new material design strategies that can tailor the recovery speed and recovery ratios for more controlled actuation.